The present invention relates to the field of lithography and more specifically to measuring profiles of photoacid patterns in chemically amplified photoresists for use in the semiconductor industry.
The use of chemically amplified photoresists (CARs) is common and increasingly important to the semiconductor industry. CARs continue to be developed in response to the increasingly demanding requirements of production lithography. A variety of acid catalyzed chemically amplified resist compositions have been used and continue to be developed. With chemically active resists, the radiation pattern incident at the wafer is recorded by a photogenerated catalyst, typically a strong Bronsted acid, which is produced by the photolytic decomposition of a photoacid generator (PAG) compound incorporated into the resist matrix. The photoacid is activated by a post exposure bake (PEB) to catalyze multiple chemical reactions in the resist matrix and thereby alter the dissolution rate. This process is called chemical amplification. The resist is then developed with the spatially dependent dissolution rate defining the ultimate pattern.
Exposure of a resist film to a radiation pattern during lithography produces what is known as the aerial image. The aerial image is transferred into the film as a catalyst image (i.e., a pattern of varying acid concentration) through photolysis of the PAG. The catalyst image is then transferred into a solubility image during the PEB by means of thermally activated catalysis. The term latent image is often used to describe either the catalyst image or the solubility image. Finally, the solubility image is transferred into patterned resist in the development step (i.e., the unexposed resist material is removed for the negative-tone case or the exposed resist material is removed for the positive-tone cse). The patterned resist is used as a local mask for the processing of the wafer.
The semiconductor industry requires that modem lithography use higher energy exposure sources to obtain greater spatial resolution in the aerial image. In the lithography process, the exposure time is of economic significance. A shorter exposure time will result in higher wafer throughput and lower production costs. The photospeed of CARs is enhanced by the amplification process during PEB. However, with this enhancement of photospeed comes the ability of the photoacid to diffuse from the exposed areas into the unexposed areas during PEB. This results in the blurring of the solubility image, which in turn leads to the blurring of the patterned resist. There is a need to both monitor and control photoacid location and activity.
The measurement of diffusion of the photogenerated catalyst in CARs is of major importance for the process control of the image formation of almost all of the lithographic techniques used today in the manufacture of microelectronic devices, including for example, integrated circuits and semiconductors. The diffusivity of an acid is measured by a quantity called D, the diffusion coefficient, which has units of length squared/time. The diffusion length (L) of an acid is given by the square root of 2 Dt, where t is the time elapsed during diffusion. Typical diffusion lengths are on the order of a few tens of nanometers. Because diffusion limits the lithographic resolution, it is a fundamental factor in the performance of CARs. Base additives are commonly used in resists to neutralize the acid and control diffusion. With the continued decrease in the dimensions of lithographic features, the ratio of the diffusion length to feature size continues to increase and the modeling and control of diffusion are becoming even more crucial issues in the design of resists and the optimization of processing conditions. The need for and the importance of measurement of diffusion of the photogenerated catalyst in chemically amplified resists is well established. Acid diffusion during post-exposure bake can markedly affect crucial dimensions and line width variation in semiconductors. The assessment of the photogenerated acid in photoresists is very significant in the manufacture of semiconductors, especially in view of increasing demand for higher circuit density in microelectronic devices. The present invention addresses this assessment need.
Precise control of the spatial distribution of photoacid during lithographic processing is paramount for maximizing lithographic resolution and minimizing critical dimension variation. An example of exercising this control is the focusing of the aerial image at the wafer. Another example is the design of resist compositions and optimization of processing conditions to minimize the diffusion of the acid from exposed to unexposed areas during PEB. Yet another example is the use of base additives to neutralize residual acid in the unexposed areas. In each of these examples, the objective is to maximize the sharpness of the photoacid concentration profiles. As the dimensions of lithographic features continue to decrease, the modeling, control, and monitoring of photoacid generation and diffusion are becoming even more crucial issues in the design and optimization of resist compositions and lithographic processes.
Photoacid distribution is generally inferred from developed patterns. However because developed patterns represent the convolution of each and every lithographic process, it is not possible to determine the photoacid distribution at each stage and hence unambiguously extract fundamental resist chemistry parameters or characterize individual processes. Furthermore, in many cases it may be desirable to inspect the outcome of a particular process before proceeding to the next step. Several methods of latent image detection have been developed in response to this problem. These include atomic force microscopy, thermal probe microscopy, photon tunneling microscopy, infrared microscopy, and fluorescence microscopy of resist doped with a pH indicator dye. The first four methods rely on contrast mechanisms resulting from variations in topography, thermal properties, refractive index, or polymer chemistry, which are essentially the result of variation in dissolution rate achieved after PEB. Fluorescence techniques, are unique by virtue of their spectroscopic, i.e., chemical, sensitivity and hence provide the ability to detect latent images before, as well as after, PEB.
At the concentrations reported to date for photoresists doped with pH-dependent dye that fluoresces in the presence of acid when exposed to radiation, the distance between dye molecules is much less than the size of the sampling area of the excitation light. These concentrations yield a continuum of acid detection across the field of view of the microscope. However, because of the limitations of optical (even near-field) microscopy, these techniques do not provide sufficient spatial resolution to detect acid concentration variations on length scales relevant to critical dimension control (i.e., less than 20 nm). The present invention addresses this need and limitation of the art.
Fluorescence detection of single molecules is a rapidly expanding field of contemporary research. Molecules have been isolated and studied in various environments, including polymer films and the measurement of acid concentration using pH sensitive single molecules embedded in an aqueous gel. Because an isolated fluorescent molecule is essentially an optical point source, its image traces out the microscope point spread function (PSF), generally a symmetric peaked function whose width is the resolution of the microscope. It has been demonstrated that the error in the measurement of the position of a signal of this type can be much less than the signal width, and is in fact only limited by the signal-to-noise ratio of the measurement. It is possible to localize single molecules to accuracies of tens of nanometers using only far-field microscopy.
In view of the previous discussion of demands and limitations in the semiconductor industry, it can be seen that there is a need to assess photogenerated acid having catalytic function in chemically amplified photoresists. Accordingly, an object of this invention is to provide a method for the measurement of photogenerated acid patterns in chemically amplified photoresists. It is also an object of this invention to provide a method for measurement of such photogenerated acid with accuracy at the nanometer level, preferably on the order of 10 nm, by means of localization of single, pH sensitive fluorescent molecules.
These and other objects and advantages of the invention and equivalents thereof, are described and provided in the drawings and descriptions that follow and manifest in the appended claims.
The present invention discloses a novel spectrofluorometic method of imaging photogenerated acid catalysts in chemically amplified photoresists. The invention comprises a method based on single molecule probing of acid concentration to measure the diffusion or profiles of photogenerated acids in chemically amplified resist films. The method of the invention measures at the molecular level fluorescent light emitted from pH-dependent dyes or fluorophores that are doped into chemically active resist films at low levels (i.e., ppb range, preferably 100 ppb or less) and are responsive to photogenerated acid catalysts produced by photolytic decomposition of photogenerator compounds in resist matixes. The invention also provides chemically amplified photoresist compositions that comprise low levels of pH-dependent fluorophores. Various resist formulation, various photacid generator compounds, and various pH-dependent fluorophores may be employed in the invention.